Question

I have an ArrayList<CustomClass> inside of ParentClass which I have written to a file using Gson.toJson(). However when I try to de-serialize the JSON using Gson.fromJson() I only get 1 element of the ArrayList<CustomClass>.

For example I will do the following

public class ParentClass {
    private ArrayList<CustomClass> myList = new ArrayList<CustomClass>();

    private GrandParentClass nested;

    public ParentClass() {
        myList.add(new CustomClass("adsf"));
        myList.add(new CustomClass("fdsa"));

        nested = new GrandParentClass();
    }

    public int arraySize() {
        return myList.size();
    }
}

public class GrandParentClass {
    private ArrayList<OtherCustomClass> myList = new ArrayList<OtherCustomClass>();

    public GrandParentClass() {
        myList.add(new CustomClass("asdfasdf.."));
        myList.add(new CustomClass("fdsafdsa..."));

    }

    public int arraySize() {
        return myList.size();
    }
}

Then when I instantiate a new instance of ParentClass, I use the following to write it to a file.

ParentClass pc = new ParentClass();
Gson gson = new Gson();
String writeThis = gson.toJson(pc); // Produces a perfect JSON reflection myList

FileOutputStream fos = new FileOutputStream(new File("writeto.json"));
fos.write(writeThis);
fos.close();

The JSON object is written in plain text to the .json file

FileInputStream fis = new FileInputStream(new File("writeto.json"));
char c;
StringBuffer sb = new StringBuffer();
while ((c = fis.read()) != -1)
    sb.append((char) c);

//Now this is where I only get 1 element of the ArrayList
Gson gson = new Gson();
ParentClass pc = gson.fromJson(sb.toString(), ParentClass.class);

Log.i("SIZE", "Size is " + pc.arraySize()); // Log output: 'Size is 1'

Now, even though I have verified that there are indeed two elements in the ArrayList in the JSON file, only 1 gets loaded into the object using fromJson.

I am serializing these just fine, I would however like to deserialize the ArrayList<OtherCustomClass> inside of the GrandParentClass, which is inside of the ParentClass, in one fell swoop.

Basically I want to serialize ArrayLists nested possibly 3 or 4 layers down in this object heirarchy, and deserialize them into 1 ParentClass that contains these nested ArrayLists<?>. How would this be accomplished?

Thanks

Answer 1

You need to do some additional work when you want to deserialize a collection when generics are involved. This is explained here.

However, I am not sure how this applies to your case, since you have the collection “nested” within a top-level non-generic class.